THURSDAY, June 20 (HealthDay News) -- Americans' vitamin D levels vary throughout the year, peaking in August and bottoming out in February, a new study shows.

The findings will contribute to understanding the vitamin's role in seasonal illnesses, according to the researchers from the University of California, Irvine, and the Mayo Clinic.

Vitamin D, which is produced by the skin when exposed to sunlight, helps bones absorb calcium and can protect against osteoporosis. It's also believed to play a role in seasonal illnesses such as the flu.

For this study, the researchers measured vitamin D levels in 3.4 million blood samples collected weekly in the United States between July 2006 and December 2011. The results appear in the journal PLoS One.

"Even with food fortification, vitamin D levels in the population show a high level of seasonality due to the influence of sunlight," study first author Amy Kasahara, a UC Irvine graduate student in public health, said in a university news release.

"In this study, we have shown that vitamin D levels lag the solar cycle, peaking in August and troughing in February," she said.

The connection between seasons and vitamin D levels has been known for some time, but this study provides more precise data about vitamin D levels at different times of the year.

"Our analysis, combined with other data, will help contribute to understanding the role of vitamin D in all seasonal diseases, where the simple winter/spring/summer/fall categories are not sufficient," study senior author Andrew Noymer, an associate professor of public health at UC Irvine, said in the news release.

Vitamin D is found in foods such as egg yolks and oil-rich fish such as mackerel, salmon, sardines and herring. Milk and cereal often are fortified with vitamin D. Another option to boost your vitamin D intake is to take supplements.